Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes characteristics of the earth formations traversed by the borehole, in addition to data relating to the size and configuration of the borehole itself. Oilfield operators use this information to locate and determine the potential yield of hydrocarbon reservoirs, and to place and configure wells in an optimal manner.
Oilfield operators often face the question of when to stop drilling. They often wish to avoid drilling too far into a subsequent formation. For example, when setting casing to line the borehole, the driller may desire to have the casing anchored in a cohesive bed rather than a highly fractured or sandy bed. In other circumstances, the driller may wish to anchor the casing in the bed above the hydrocarbon reservoir, and later may wish to avoiding drilling through the bottom of the reservoir. One method for determining stop points employs a pilot borehole which is logged and used as a reference for the current borehole. Operators can obtain logs from each borehole using logging while drilling (LWD) resistivity tools such as those disclosed in U.S. Pat. No. 6,359,438 to Bittar, entitled “Multi-Depth Focused Resistivity Imaging Tool for Logging While Drilling Applications”, and U.S. Pat. No. 5,339,037 to Bonner et al., entitled “Apparatus and Method for Determining the Resistivity of Earth Formations”.
To determine the current bit position relative to formation boundaries, the driller correlates logging measurements from the current borehole with the pilot borehole log. However, this approach may be unreliable due to the dip and thickness variation of the earth formations between the boreholes.
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that the drawings and detailed description thereto are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the spirit and scope of the appended claims.